Highly corrosion resistant nickel-chromium-molybdenum alloy with improved resistance o intergranular corrosion



Aug. 31, 1965 E. scHEn. ETAL 3,203,792

HIGHLY CORROSION RESISTANT NICKEL-CHROMIUM-MOLYBDENUM ALLOY WITHIMPROVED RESISTANCE TO INTERGRANULAR CORROSION Filed Aug. 5l. 1964 en AFI G. 3

linear decrease inthickness m /year o 2o 4e|6IQ 8o |00 percent by weightof HCOOH (boiling point) thickness of alloy sheeting in mm lNvENToRs:ERicH scHEn. DEcEAsED BY MARGARET scHE||.,LEGAL REPRESENT'ATIVE IMMANUELcLAss HUBERT @RAI-:PEN

Si content in percent ATT'YS United States Patent O 3,203,792 HIGHLYCORRG'SIN RESSTANT NTCKEL- CHROMIUM-MGLYBDENUM ALLOY WITH fliMlRGVEDRESISTANCE T INTERGRAN- ULAR CORROSHBN Erich Scheil, deceased, late ofStuttgart, Germany, by Margarete Scheil, heir and legal representativeof minor heirs, Stuttgart, Germany, and Immanuel Class and HubertGraefen, Ludwigshafen, Rhineland, Germany, assignors to Badische Anilin-& Soda-Fabrik Aktiengesellschaft, Ludwigshafen, Rhineland, Germany`Filed Aug. '31, 1964, Ser. No. 393,827 Claims priority, applicationGermany, 'Api'. '1, 1961,

4 claims. (l. *7s-171) 54 to 60% nickel, 14.5 to 16.5% chromium, 15 to17% molybdenum, 4 to 7% iron and a maximum of 0.1% carbon and of 1%silicon.

If desired, the alloy can also contain from 3 to 4.5%

ltungsten and up to 3% cobalt.

Alloys with the above mentioned composition, however, are heterogeneousin the state of equilibrium and, therefore, have low corrosionresistance. To impart to them favorable structural characteristics theymust be subjected to a heat treatment which consists of annealing at atemperature above 1,200 C. with a subsequent water quench. Upon furtherheating to 600 to 1,100o C. the supersaturation produced at roomtemperature in the jstructure brings about the formation of new phases,which preferentially .appear at the grain boundaries. These types ofcompounds are rich in chromium and molybdenum and are designated assigmaphase. However,

Through the impoverishment of the grain boundary areas in chromium andmolybdenum, these alloys show high susceptibility to intergranularcorrosion, whereas the precipitates themselves have reduced workabilityas a result, which can cause complete embrittlement of the material.This high susceptibility to grain decomposition with reduced toughnessin the heat-affected zone also appears after welding and can only beeliminated by an additional solution annealing and water quenchtreatment. This procedure, however, is technically not always feasible'because of the size of certain of the equipment and the dif- 'iicultiesinvolved in annealing and quenching the product,

especially because of the high temperature which has to be employed. Forthese reasons, nickel-chromium-molybdenum alloys cannot be used in manyinstances.

It is an object of the present invention to provide-nickel-chromium-molybdenum alloys with improved resistance tointergranular corrosion.

Another object of the invention is -to provide alloys `which when usedfor the manufacture of apparatus need not be subjected to a heattreatment after welding.

ln general, the present invention comprises the discovery that a greatlyimproved nickel-chromium-molybdenum alloy can be produced if the siliconcontent of the alloy is maintained at an extremely low level. In allVother phases with a similar composition are also formed.

for solution annealing. Vbut silicon-free alloy (nickel 58.8%, chromium15.5%,

3,203,792 Patented Aug. 3l, 1,965

ICC

events, the silicon level should be less than 0.2%, and

preferably should be from 0 to 0.1%, and more preferably from 0 to0.012%. Ordinarily, the above described nickel-chromiummolybdenum alloyshave a silicon content of 0.5 to 1%. It has been suggested in the pastthat the presence of silicon is needed for deoxidation of the alloy andto improve the forgeability of the alloys. Silicon is also almost alwaysincorporated in the alloy as an impurity due to the fact thatsilicon-containing materials are used as deoxidation agents.

It has been found that by maintaining the silicon content of thenickel-chromium-molybdenum alloy at an extremely low level theprecipitation rate of the phases mentioned above, and especially the-sigma-phase, is substantially decreased. Likewise, the solubility ofchromium and molybdenum in the nickel-rich matrix is increased so thatlower temperatures are needed for solution annealing. Likewise, alloyscontaining from 0 to 0.1% silicon, in most cases, especially if thethickness does not exceed 10 mm., need not be subjected to a furtherheat treatment after welding.

According to the invention, a highly corrosion and heat resistantnickel-chromium-molybdenum alloy is obtained with improved resistance tointergranular corrosion by using: a nickel content of 40 to 65%,preferably of 55 to 60%, part of the nickel, up to a maximum of 20%, ifdesired being replaced by cobalt; a chrome content of 14 to 26%,preferably 22 to 25%; a molybdenum content of 3 to 18%, preferably 14 to17%; an iron content of O to 30%, preferably 0 to 7%; a tungsten contentof 0 to 5%; a carbon content of not more than 0.1%; a manganese contentof up to 3%; a silicon content of from 0 to less than 0.2%; as well as aphosphorus and sulfur content totaling not more than 0.1%, if these areproduced from corresponding metals or master alloys which are free ofsilicon and by subsequent deoxidation with a silicon-free alkaline-earthmetal, preferably magnesium, or a silicon-free alkaline-earth metalmaster Ialloy, prefererably a silicon-free nickel-alkaline-earth metalmaster alloy, or with a silicon-free titaniuml master alloy.

According to the invention, instead of silicon, an alkaline-earth metal,for instance magnesium, or a siliconfree alkaline-earth metal masteralloy, is used as deoxidation agent, whereby the disadvantageousproperties of the silicon in the alloy are avoided. Alloys deoxidizedwith magnesium, as an example, show much more sluggish precipitates.Moreover, these cover the grain boundaries only very slowly.

Therefore, and this is the special and unexpected advantage of the useof the alloys produced according to the invention, heat treatment is nolonger necessary after the welding of sheets with a wall thickness up toe.g. 10 mm., in order to obtain a structure with a high corrosionresistance, especially to intergranular corrosion.

'In this way it is possible to weld vessels of any size with- 'out thenecessity of a heat treatment and without encountering the greatdiiculties of annealing and quenching. Moreover, solution annealing doesnot require as high temperatures with silicon-free alloys as with thosecontaining silicon.

For instance, a commercial alloy containing 56.8% nickel, '15.8%chromium, 16.5% molybdenum, 3.4% tungsten, 5.2% iron, 0.95% manganese,0.052% carbon and 0.61% silicon, requires a temperature of 1,220 C. Analmost equally composed,

molybdenum 17.0%, iron 3.8%, tungsten 3.1%, manga- 4nese 0.85%, carbon0.04% and silicon 0.01%), could 'be solution annealed at 1,12 0 C. andquenching with water was not necessary as cooling in air was sutiicient.

Instead of magnesium, a mixture of silicon-free calcium withsilicon-free strontium or silicon-free barium can be used fordeoxidation. Other deoXidation media are however not suitable. Aluminumhas proved to be very unfavorable.

Of the silicon-free alloys investigated, the alloys with the followingcomposition proved to be especially favorable: 55 to 60% nickel, 22 to25% chromium, 14 to 17% molybdenum, iron 2%, manganese l%, silicon 0 to0.19% and carbon 0.08%. The range of dangerous precipitates is inaddition reduced by the increase of the chromium content as comparedwith alloys of normal composition Without silicon. They offer,therefore, increased safety to intergranular corrosion, especially afterwelding.

In the event solution annealing is necessary for some reason, this canbe done preferably at 1,150 C. and does not require a water quench as isthe case with conventional alloys. For instance, 3 mm. thick sheets of asilicon-free nickel-chromium-molybdenum alloy having the followingcomposition: nickel 61.3%, chromium 22.6%, molybdenum 14.0%, iron 1.3%,manganese 0.84%, silicon 0.012%, carbon 0.04%, have shown no grainboundary segregation after welding and quenching in still air. Incontrast to this, commercial sheets of a thickness of 3 mm. with thefollowing analysis:

Nickel 56.4%, chromium 15.3%, molybdenum 16.1%, tungsten 3.4%, iron5.2%, silicon 0.61%, manganese 0.95%, carbon 0.05%

Nickel 59.1%, chromium 16.6%, molybdenum 16.9%, iron 5.8%, manganese0.9%, carbon 0.06%, silicon 0.58%

Table 1 In the tests the following alloys were compared:

Alloy Ni Cr Mo Fe Mn C Si Mg MeltA (a commercia1auoy) 56.3 15.1 13.3 5.30.30 0.05 0.61 eltv 59.4 24.1 14.1 1.4 0.85 0.03 0.04 0.11

As is apparent from FIGS. l to 3 of the attached drawing the resistanceagainst general corrosion (uniform attack) after solution annealing ofan alloy prolduced from Melt B was far greater than an alloy producedfrom Melt A. This is due to the increased content of chromium andreduced content of silicon in the alloy of Melt B.

The resistance of the alloys to intercrystalline corrosion rises rapidlyas the Si-content of the alloy drops. As -is apparent from FIG. 4 of thedrawing, the corrosion effect is negligible up to an Si-content of-0.1%. Intercrystalline corrosion occurs as a result of the formation ofprecipitates in the grain boundaries, the speed of precipitationdepending on the Si-content. Such precipitates are particularlypronounced in the heat-affected zones of the Welding seams. Therefore,these zones are particularly endangered due to their susceptibility tointercrystalline corrosion. The degree of their susceptibility -isdependent on the duration and intensity of heating.

The graph set out in FIG. 4 shows the dependency of the thickness ofalloy sheeting on its Si-content, i.e., what thickness such sheetingmust have to insure that no intercrystalline corrosion occurs when suchsheeting is not heated after welding. As mentioned above the impairmentis negligible with an Si-content of up to 0.1%, but becomes severe evenwith as low an Si-content as 0.2%. The graph indicates the maximumthickness up to which alloy sheeting can be welded without the Weldingseam having to be heat-treated. The area between the curve and theordinate pertains to sheeting which will be adequately resistant tointercrystalline corrosion without heat treatment. On the other hand, ifthe values for Si-content and thickness fall outside this area, thismeans that the sheeting must be subjected to a heat treatment afterwelding.

When the two alloys described above are welded it is found that thealloy produced from Melt A is severely affected at the edges of itswelding seam and especially in the heat-affected zones. In contrast, thewelding seam in the heated zones remains unaffected with respect to thealloy of Melt B. As the Si-content of the alloy is increased, theprecipitation zone is shifted toward higher temperatures so that veryhigh solution annealing temperatures are required. In the case of thealloy of Melt A an annealing temperature of 1,220 C. is required.Moreover, due to the high speed of precipitation which results as theSi-content is increased, a quench with water is necessary to insure thatthe critical temperature range will be rapidly passed. In contrast,alloys having a maximum Si-content of 0.1% only require a temperature of1,150 C. for solution annealing and can be cooled 1n air.

The differences in the properties of the alloys which occur as afunction of the Si-content of the alloy also is evident with respect tothe hardness and impact strength of the alloys as is evident from Table2 which shows that the alloys become more substantially brittle as theSicontent is increased.

The specimens used for the impact tests described in Table 2 were pieces4 by 3 by 27 mm.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

We claim:

1. A nickel-chromiurrr-molybdenum alloy having high corrosion and heatresistance properties, said alloy consisting essentially of from 40 to65 nickel; up to 20% cobalt, the total amount of nickel and cobalt beingfrom 40 to 65%; from 14 to 26% chromium; from 3 to 18% molybdenum; up to30% iron; up to 5% tungsten; up to 0.1% carbon; up to 3% manganese; anda combined phosphorus and sulfur content of up to 0.1%, said alloycontaining less than 0.2% silicon.

2. A nickel-chromium-molybdenum alloy having high corrosion and heatresistance properties, said alloy consisting essentially of from 55 to60% nickel; up to 20% cobalt, the total amount of nickel and cobaltbeing from 55 to 60%; from 22 to 25% chromium; from 14 to 17%molybdenum; up to 7% iron; up to 5% tungsten; up to 0.1% carbon; up to3% manganese; a combined phos- 65 Q3 phorus and sulfur content of up to0.1%; and up to 0.1% molybdenum, about 1% iron, about 1% manganese, asilicon. maximum of 0.03% carbon, a maximum of 0.04% silicon,

3. A nickel-chromium-molybdenum alloy having high balll nickel.

corrosion and heat resistance properties, said alloy consistingessentially of from 55 to 60% nickel; from 22 to 25% 5 Referegs Cmd bythe Examner chromium; from 14 to 17% molybdenum; up to 2% iron; UNITEDSTATES PATENTS up to 0.08% Carbon; up t0 1% manganese; and up t02,840,469 6/58 Gresham etal 75-171 0.012% silicon. 2,959,480 11/60 Flint75-171 4. A nickel-chromium-molybdenum ailoy having high corrosion andheat resistance properties, said alloy con- 10 DAVID L RECK, PIWUYExaminersisting essentially of about 24% chromium, about 14% HYLANDBZQT, Examiner.

1. A NICKEL-CHROMIUN-MOLYBDENUM ALLOY HAVING HIGH CORROSION AND HEATRESISTANCE PROPERTIES, SAID ALLOY CONSISTING ESSENTIALLY OF FROM 40 TO65% NICKEL; UP TO 20% COBALT, THE TOTAL AMOUNT OF NICKEL AND COBALTBEING FROM 40 TO 65%; FROM 14 TO 26% CHROMIUM; FROM 3 TO 18% MOLYBDENUM;UP TO 30% IRON: UP TO 5% TUNGSTEN; UP TO 0.1% CARBON; UP TO 3%MANGANESE; AND A COMBINED PHOSPHORUS AND SULFUR CONTENT OF UP TO 0.1%,SAID ALLOY CONTAINING LESS THAN 0.2% SILICON.